Recently, development of multi-functional devices which are low-priced but have a high reliability has noticeably progressed in electronics industries. Accordingly, high-function and high-density devices and elements have been realized and a demand for light-weighed, thin-formed and small-sized devices having a high reliability and a multi-function is increasing. As such, development of a novel and improved technique for fitting such devices and elements in electronics apparatus has become increasingly important. In particular, development of small-sized and diversified IC package parts is an especially important theme in this technical field. With progress of the technique of fitting electronic devices and elements, formation of fine pin pitches capable of satisfying the demand for multi-pin structure in small-sized IC packages is desired.
TAB is one having a plurality of bonding fine metal lead patterns as formed on an electrically insulating tape-shaped synthetic resin film substrate such as polyimide tape substrate. Since TAB has test pads, it is possible to find out any bonding failure or chip disorder after bonding of chips but before fitting of substrate to TAB. Additionally, the size of the IC pad to be applied may be much small-sized as compared with wire bonding and therefore formation of a more multi-pined structure is possible. That is, TAB is characterized by the noted advantages.
TAB is classified into three kinds: one-layer TAB, two-layer TAB and three-layer TAB. One-layer TAB is composed of only a patterned metal tape such as a patterned copper foil tape. Such a one-layer TAB has a poor mechanical strength since the thickness of the metal layer itself is at most about several ten microns. Additionally, the number of pins to be applied to the one-layer TAB is limitative. Therefore, the one-layer. TAB is not suitable for large-scaled IC or the like devices. In order to overcome the drawback of the one-layer TAB, a three-layer TAB has been developed in which a metal foil is laminated on a synthetic resin film via an adhesive and the metal foil is patterned. However, the three-layer TAB has a drawback that insulation between pins could not sufficiently be ensured even through a highly insulating synthetic resin film such as a polyimide resin film is used as a substrate, because of the influence of the adhesive to be used as the interlayer.
Two-layer TAB is manufactured by directly forming a metal layer on the surface of an insulating synthetic resin film substrate by sputtering, vapour deposition, plating or the like metal-coating method without using an adhesive followed by patterning the thus-formed metal layer. As the two-layer TAB does not use any adhesive in forming the metal layer on the surface of the substrate, it is free from problems of electric insulation and can be used stably. Therefore, the use of the two-layered TAB is considered hopeful.
Formation of leads on the surface of the two-layer TAB can be effected by an additive method in which a photo-resist is coated on a subbing metal layer as coated on the surface of an insulating synthetic resin substrate such as a polyimide resin substrate by dry surface-treatment such as sputtering or vacuum evaporation coating or by wet surface-treatment such as chemical plating, as mentioned above, the thickness of the photo-resist being larger than the thickness of the leads to be formed, then a specifically defined resist is formed by exposure and development, and thereafter leads are finally formed by electric plating. The leads can also be formed by a subtractive method in which a metal layer is previously formed on the above-mentioned subbing metal layer in the same thickness as leads, then a resist pattern is formed on the layer, and thereafter the intended leads are finally formed by etching.
For continuously bonding such two-layer TAB's to IC chips, it is necessary to form holes for conveying the tape (hereinafter referred to as a "sprocket hole"), holes for exposing the top of the lead so that the exposed lead is bonded to an IC chip (hereinafter referred to as a "device hole"), holes for connecting the bottom of the lead to an external circuit (hereinafter referred to as an "OLB" hole), and further holes for inserting a guide pin thereinto in order that the top of the lead may accurately be bonded to an IC chip (hereinafter referred to as a "tooling hole), these holes being provided through the insulating chemical resin substrate by chemical dissolution.
For dissolution of the insulating resin, there may be employed a wet method of dissolving the resin with an insulating resin-dissolving liquid as well as an optical method of using laser rays.
It has already been confirmed that such two-layer TAB's manufactured by combination of the above-mentioned various methods have a high-level potency as mentioned above. However, because of further elevation of the fitting density of electronic parts and further elevation of the performance speed of electronic apparatus, development of further high-leveled two-layer TAB's having a function of preventing errors by cross talk and a function of impedance matching is desired. As one example of two-layer TAB's provided with such functions, there has been proposed a two-layer TAB in which a ground metal layer is formed on the opposite surface to the surface having leads thereon and via holes are holed through a resin substrate so that a part of the leads are electrically connected with the ground metal layer via the via holes.
A method of preparing the proposed two-layer TAB will be briefly mentioned below. First, a metal layer such as a copper layer is formed on both surfaces or one surface of an insulating resin substrate such as a polyimide substrate without using an adhesive. For forming the metal layer, conventional methods which include a dry metal layer-forming method such as a sputtering method and a wet metal layer-forming method such as a chemical plating method are used. Next, a photo-resist layer is formed on the both surfaces of the substrate; a photo-mask having a pattern corresponding to the determined leads is applied to the top surface of the substrate while a photo-mask corresponding to the determined via holes is applied to the back surface of the same; and thus the substrate is exposed and developed so as to form the determined resist patterns on the both surfaces.
Formation of the leads may be effected, using either the above-mentioned additive method or the above-mentioned additive method in accordance with the thickness of the metal layer as previously formed on the back surface of the substrate. Formation of the via holes may be effected by dissolving and removing the exposed parts of the insulating resin as exposed in accordance with the resist pattern on the back surface of the substrate, using a dissolving liquid, when the metal layer has previously been formed on only one surface of the substrate. On the other hand, where the metal layer has previously been formed on both surfaces of the substrate, formation of the via holes may be effected in such a way that a metal layer pattern in which only the parts corresponding to the intended via holes are exposed is formed on the back surface of the substrate by resist pattern treatment, and thereafter the parts where the insulating resin is exposed are dissolved in the same manner as mentioned above to form the intended via holes.
Next, a thin metal layer is formed on all the back surface of the substrate including the side surfaces of the formed via holes by a dry or wet metal layer-forming method, and thereafter a resist layer is formed on the thin metal layer either immediately or after lamination of a metal plate layer on the thin metal layer by electric plating or the like. Then, a photo-mask having a desired hole pattern, including device holes, OLB holes, sprocket holes, and tooling holes is applied to the back surface of the substrate as coated with the resist layer, and the substrate is thereafter exposed and developed to form the desired resist pattern.
Where the resist layer has been formed immediately after formation of the thin metal layer on the back surface of the substrate, the above-mentioned additive method may be employed to form a ground metal layer having the desired pattern. On the other hand, where the resist layer has been formed after lamination of the metal plate layer over the thin metal layer formed on the back surface of the substrate, the above-mentioned subtractive method may be employed to form the same.
The above-mentioned process of forming the two-layer TAB having the above-mentioned construction has two exposure steps. In the second exposure step, in particular, it is necessary to effect accurate alignment between the resist layer as formed on the back surface of the substrate and the photo-mask to be applied to the resist layer. However, provision of alignment marks for attaining accurate alignment between the positions of the formed via holes and the relative positions of other various holes in the photo-mask is generally unknown.
Under the situation, development of a method for attaining accurate alignment between the positions of the via holes as formed on the back surface of a substrate and the relative positions of other various holes to be formed after formation of the via holes is desired. The object of the present invention is to provide photo-masks capable of easily effecting such accurate alignment in manufacture of a two-layer TAB.